RNA interference (RNAi) is the mechanism of sequence-specific, post-transcriptional gene silencing initiated by double-stranded RNAs (dsRNA) homologous to the gene being suppressed. dsRNAs are processed by Dicer, a cellular ribonuclease III, to generate duplexes of about 21 nt with 3′-overhangs (small interfering RNA, siRNA) which mediate sequence-specific mRNA degradation. In mammalian cells siRNA molecules are capable of specifically silencing gene expression without induction of the unspecific interferon response pathway. Thus, siRNAs have become a new and powerful alternative to other genetic tools such as antisense oligonucleotides and ribozymes to analyze gene function. Moreover, siRNA's are being developed for therapeutic purposes with the aim of silencing disease genes in humans.
RNA silencing refers to a group of sequence-specific regulatory mechanisms (e.g. RNA interference (RNAi), transcriptional gene silencing (TGS), post-transcriptional gene silencing (PTGS), quelling, co-suppression, and translational repression) mediated by RNA silencing agents which result in repression or “silencing” of a corresponding protein-coding gene. RNA silencing has been observed in many types of eurkayotes, including humans, and utility of RNA silencing agents as both therapeutics and research tools is the subject of intense interest.
Several types of small (˜19-23 nt), noncoding RNAs trigger RNA silencing in eukaryotes, including small interfering RNAs (siRNAs) and microRNAs (miRNAs, also known as small temporal RNAs (stRNAs)). Recent evidence suggests that the two classes of small RNAs are functionally interchangeable, with the choice of RNA silencing mechanism (e.g. RNAi-mediated mRNA cleavage or translational repression) determined largely by the degree of complementarity between the small RNA and its target (Schwarz and Zamore, 2002: Hutvágner and Zamore, 2002: Zeng et al., 2003; Doench et al., 2003). RNA silencing agents with a high degree of complementarity to a corresponding target mRNA have been shown to direct its silencing by the cleavage-based mechanism (Zamore et al., 2000; Elbashir et al., 2001a; Rhoades et al., 2002; Reinhart et al., 2002; Llave et al., 2002a; Llave et al., 2002b; Xie et al., 2003; Kasschau et al., 2003; Tang et al., 2003; Chen, 2003). RNA silencing agents with a lower degree of complementarity mediate gene silencing by recruiting the RISC complex to the target mRNA, thereby blocking its translation but leaving the mRNA intact (Mourelatos et al., 2002; Hutvágner and Zamore, 2002; Caudy et al., 2002; Martinez et al., 2002; Abrahante et al., 2003; Brennecke et al., 2003; Lin et al., 2003; Xu et al., 2003).
RNA silencing agents have received particular interest as research tools and therapeutic agents for their ability to knock down expression of a particular protein with a high degree of sequence specificity. The sequence specificity of RNA silencing agents is particularly useful for allele-specific silencing dominant, gain-of-function gene mutations. Diseases caused by dominant, gain-of-function gene mutations develop in heterozygotes bearing one mutant and one wild type copy of the gene. Some of the best-known diseases of this class are common neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis (ALS; “Lou Gehrig's disease”) (Taylor et al., 2002). In these diseases, the exact pathways whereby the mutant proteins cause cell degeneration are not clear, but the origin of the cellular toxicity is known to be the mutant protein.
One group of inherited gain-of-function disorders are known as the trinucleotide repeat diseases. The common genetic mutation among these diseases is an increase in a series of a particular trinucleotide repeat. To date, the most frequent trinucleotide repeat is CAG, which codes for the amino acid glutamine. At least 9 CAG repeat diseases are known and there are more than 20 varieties of these diseases, including Huntington's disease, Kennedy's disease and many spinocerebellar diseases. These disorders share a neurodegenerative component in the brain and/or spinal cord. Each disease has a specific pattern of neurodegeneration in the brain and most have an autosomal dominant inheritance. The onset of the diseases generally occurs at 30 to 40 years of age, but in Huntington's disease CAG repeats in the huntingtin gene of >60 portend a juvenile onset.
Recent research by the instant inventors has shown that the genetic mutation (increase in length of CAG repeats from normal <36 in the huntingtin gene to >36 in disease) is associated with the synthesis of a mutant huntingtin protein, which has >36 polyglutamines (Aronin et al., 1995). It has also been shown that the protein forms cytoplasmic aggregates and nuclear inclusions (Difiglia et al., 1997) and associates with vesicles (Aronin et al., 1999). The precise pathogenic pathways are not known.
Huntington's disease (and by implication other trinucleotide repeat diseases) is believed to be caused, at least in part, by aberrant protein interactions, which cause impairment of critical neuronal processes, neuronal dysfunction and ultimately neuronal death (neurodegeneration in brain areas called the striatum and cortex).
In the search for an effective treatment for these diseases, researchers in this field emphasized understanding the pathogenesis of the disease and initially sought to intercede at the level of the presumed aberrant protein interactions. However, there is no approved treatment for Huntington's disease or other trinucleotide repeat diseases. Accordingly, therapeutic RNA silencing agents capable of silencing Huntingtin proteins are of considerable interest.